Modern wind energy plants provide control of power and rotor speed by changing the aerodynamic force that is applied to the rotor. Usually, this is performed by changing the pitch of the rotor blades. The operation of a wind energy plant or a wind turbine may be divided into two regimes. In a first regime, at lower wind speeds, no pitch activity during operation of the power plant is necessary. The rotor blade is fixed or held in an optimal aerodynamic position. In a second regime, at higher wind speeds of typically more than 12 ms or in turbulent wind situations, pitch activity, i.e. a frequent adjustment of the pitch of the rotor blades, is necessary during operation of the wind turbine. A transition between the two regimes will occur e.g. for freshening winds.
However, most of the time, the rotor blade is in either of two fixed positions relating, for example to lower wind speeds and to the situation, when the wind turbine is shut down. The rotor blade is then held in a specific position by brake forces applied to the shaft of the pitch drive. The pitch drive is then switched off. The brakes are fail-safe brakes. This means that they open when power is applied to them and closed when the power supply is interrupted. The advantage of this prior art configuration is that the brakes can be dimensioned for rather small brake torques due the gear and gear transmission ratio.
One of the drawbacks of this solution is that the play caused by the annular gear of the bearing meshing with the drive bevel of the gearbox of the rotor blade allows the rotor blade to move even when the brake is closed with maximum torque. This results in an undesired wear and abrasion of these components.
If the pitch drive or other components for rotating the rotor blade are exchanged, the previously described brake system fails and the rotor blade cannot be fixed until the maintenance or service operation is completed.
Another drawback is that the brake has to be dimensioned in accordance with the maximum torque resulting from the turbine design. The nominal torque of the brake is then often greater than the peak torque that can be achieved with the pitch drives. If the brake is closed due to a failure, the rotor blade cannot be rotated anymore and the blade remains in the position that was assumed when the failure occurred.
In order to avoid this problem, the pitch drive has to be configured to have even greater torque in order to overcome the brake force. This increases the dimensions and costs for the pitch drives.
German patent application DE 10 2009 008607 discloses a locking mechanism for fixing the rotor blade in any arbitrary position. However, once the rotor blade is locked, it cannot be moved anymore.
Other possibilities in order to overcome the above mentioned drawbacks are redundant brake systems, where two brakes are provided each of which applies half the maximum necessary torque. If one of the brakes fails, the pitch drives can still rotate the rotor blade. However, redundant brake systems need more components.